Coherent light generators – Particular active media – Gas
Reexamination Certificate
2002-09-13
2004-09-28
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Gas
C372S057000, C372S058000, C372S059000, C372S060000, C372S061000
Reexamination Certificate
active
06798812
ABSTRACT:
BACKGROUND OF THE INVENTION
Electric Discharge Gas Lasers
Electric discharge gas lasers are well known and have been available since soon after lasers were invented in the 1960s. A high voltage discharge between two electrodes excites a laser gas to produce a gaseous gain medium. A resonance cavity containing the gain medium permits stimulated amplification of light which is then extracted from the cavity in the form of a laser beam. Many of these electric discharge gas lasers are operated in a pulse mode.
Excimer Lasers
Excimer lasers are a particular type of electric discharge gas laser and they have been known since the mid 1970s. A description of an excimer laser, useful for integrated circuit lithography, is described in U.S. Pat. No. 5,023,884 issued Jun. 11, 1991 entitled “Compact Excimer Laser.” This patent has been assigned to Applicants' employer, and the patent is hereby incorporated herein by reference. The excimer laser described in Patent '884 is a high repetition rate pulse laser. These excimer lasers, when used for integrated circuit lithography, are typically operated in a fabrication line “around-the-clock” producing many thousands of valuable integrated circuits per hour; therefore, down-time can be very expensive. For this reason most of the components are organized into modules which can be replaced within a few minutes. Excimer lasers used for lithography typically must have its output beam reduced in bandwidth to a fraction of a picometer. Electric discharge gas lasers of the type described in Patent '884 utilize an electric pulse power system to produce the electrical discharges, between the two electrodes. In such prior art systems, a direct current power supply charges a capacitor bank called “the charging capacitor” or “C
0
” to a predetermined and controlled voltage called the “charging voltage” for each pulse. The magnitude of this charging voltage may be in the range of about 500 to 1000 volts in these prior art units. After C
0
has been charged to the predetermined voltage, a solid state switch is closed allowing the electrical energy stored on C
0
to ring very quickly through a series of magnetic compression circuits and a voltage transformer to produce high voltage electrical potential in the range of about 16,000 volts (or greater) across the electrodes which produce the discharges which lasts about 20 to 50 ns.
Major Advances in Lithography Light Sources
Excimer lasers such as described in the '884 patent have during the period 1989 to 2001 become the primary light source for integrated circuit lithography. More than 1000 of these lasers are currently in use in the most modern integrated circuit fabrication plants. Almost all of these lasers have the basic design features described in the '884 patent.
This is:
(1) a single, pulse power system for providing electrical pulses across the electrodes at pulse rates of about 100 to 2500 pulses per second;
(2) a single resonant cavity comprised of a partially reflecting mirror-type output coupler and a line narrowing unit consisting of a prism beam expander, a tuning mirror and a grating;
(3) a single discharge chamber containing a laser gas (either KrF or ArF), two elongated electrodes and a tangential fan for circulating the laser gas between the two electrodes fast enough to clear the discharge region between pulses, and
(4) a beam monitor for monitoring pulse energy, wavelength and bandwidth of output pulses with a feedback control system for controlling pulse energy, energy dose and wavelength on a pulse-to-pulse basis.
During the 1989-2001 period, output power of these lasers has increased gradually and beam quality specifications for pulse energy stability, wavelength stability and bandwidth have also become increasingly tighter. Operating parameters for a popular lithography laser model used widely in integrated circuit fabrication include pulse energy at 8 mJ, pulse rate at 2,500 pulses per second (providing an average beam power of up to about 20 watts), bandwidth at about 0.5 pm (FWHM) and pulse energy stability at +/−0.35%.
F
2
Lasers
F
2
lasers are well known. These lasers are similar to the KrF and ArF lasers. The basic differences are the gas mixture which in the F
2
laser is a small portion of F
2
with helium and/or neon as a buffer gas. The natural output spectrum of the F
2
laser is concentrated in two spectral lines of narrow bandwidth, a relatively strong line centered at about 157.63 nm and a relatively weak line centered at about 157.52 nm.
F
2
Lasers Bandwidth
A typical KrF laser has a natural bandwidth of about 300 pm measured full width half maximum (FWHM) centered at about 248 nm and for lithography use, it is typically line narrowed to less than 0.6 pm. (In this specification bandwidth values will refer to the FWHM bandwidths unless otherwise indicated.) ArF lasers have a natural bandwidth of about 500 centered at about 193 nm and is typically line narrowed to less than 0.5 pm. These lasers can be relatively easily tuned over a large portion of their natural bandwidth using the grating based line narrowing module referred to above. F
2
lasers, as stated above, typically produce laser beams with most of its energy in two narrow spectral features (referred to herein sometimes as “spectral lines”) centered at about 157.63 nm and 157.52 nm. Often, the less intense of these two spectral lines (i.e., the 157.52 nm line) is suppressed and the laser is forced to operate at the 157.63 nm line. A known technique for suppressing the weak line is to spectrally spread the laser beam using beam dispersing optics within the resonant cavity of the laser and to spatially block the weak line at an aperture. The natural bandwidth of the 157.63 nm line is pressure and gas content dependent and varies from about 0.6 to 1.2 pm (FWHM). An F
2
laser with a bandwidth in this range can be used with lithography devices with a catadiophic lens design utilizing both refractive and reflective optical elements, but for an all-refractive lens design the laser beam bandwidth may have to be reduced to about 0.1 pm to produce desired results.
Injection Seeding
A well-known technique for reducing the band-width of gas discharge laser systems (including excimer laser systems) involves the injection of a narrow band “seed” beam into a gain medium. In one such system, a laser producing the seed beam called a “master oscillator” is designed to provide a very narrow bandwidth beam in a first gain medium, and that beam is used as a seed beam in a second gain medium. If the second gain medium functions as a power amplifier, the system is referred to as a master oscillator, power amplifier (MOPA) system. If the second gain medium itself has a resonance cavity (in which laser oscillations take place), the system is referred to as an injection seeded oscillator (ISO) system or a master oscillator, power oscillator (MOPO) system in which case the seed laser is called the master oscillator and the downstream system is called the power oscillator. Laser systems comprised of two separate systems tend to be substantially more expensive, larger and more complicated than comparable single chamber laser systems. Therefore, commercial application of these two chamber laser systems has been limited.
What is needed is a better laser design for a pulse gas discharge F
2
laser for operation at repetition rates in the range of about 4,000 pulses per second or greater, permitting precise control of all beam quality parameters including wavelength and pulse energy.
SUMMARY OF THE INVENTION
The present invention provides an injection seeded modular gas discharge laser system capable of producing high quality pulsed laser beams at pulse rates of about 4,000 Hz or greater and at pulse energies of about 5 to 10 mJ or greater for integrated outputs of about 20 to 40 Watts or greater. Two separate discharge chambers are provided, one of which is a part of a master oscillator producing a very narrow band seed beam which is amplified in the second discharge chamber. The parameters chamber can b
Hofmann Thomas
Rylov German E.
Sandstrom Richard L.
Cray William
Cymer Inc.
Ip Paul
Rodriguez Armando
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